Embodiments of the invention relate generally to tomographic imaging and, more particularly, to an apparatus and method of acquiring tomographic imaging data and reconstructing a tomographic image having improved temporal resolution.
Typically, in x-ray systems, such as computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped or cone-shaped beam toward a subject, such as a patient, a piece of luggage, or any other object of interest. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam of radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces an electrical signal indicative of the attenuated beam received by the detector element. The electrical signals are converted to digital signals and transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam from a focal point. X-ray detectors typically include a collimator for collimating x-ray beams directed toward the detector, a scintillator adjacent to the collimator for converting x-rays to light energy, and photodiodes for receiving the light energy from the scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy and discharges the light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are digitized and then transmitted to the data processing system for image reconstruction. The x-ray detector extends over a circumferential angular range or fan angle, often typically 60°.
The general terminology “CT imaging” encompasses multiple configurations. For example, configurations can include a multi-slice imaging system or a multi-detector CT (MDCT) imaging system, as examples, which may be employed for cardiac imaging. Such a system may be used to generate a cardiac image using imaging data that is obtained over a portion or phase of a cardiac cycle. Conventionally, the minimum projection angle of imaging data for image reconstruction is 180° of gantry rotation plus the x-ray detector fan angle. Thus, with a typical fan angle of 60°, the minimum projection angle or temporal aperture is 240° of projection data for image reconstruction. This projection data is said to be obtained over a “half-scan” or “short scan” range of coverage and may be reconstructed using known reconstruction techniques. The amount of time taken to obtain this half-scan projection dataset together with the reconstruction algorithm, in this conventional example, defines the temporal resolution of the imaging system. In other words, the temporal resolution is defined as the time taken to obtain minimally adequate data for image reconstruction and the data actually used in the reconstruction. In one case, short scan data is obtained for 240° of gantry rotation with some type of weighting function, as is understood in the art.
The range of angular coverage (or temporal aperture) and gantry rotational speed are thus primary factors that define temporal resolution in an MDCT scanner. In a typical single source MDCT scanner, temporal resolution is thus approximately 135 ms for a gantry rotational speed of 270 ms, and approximately 175 ms for a gantry rotational speed of 350 ms with a Parker-type weighting, for example. In many imaging applications, such temporal resolution is adequate to provide images with acceptable motion artifacts.
Due to motion of the heart during the 240° of gantry rotation when this short scan data is acquired however, the temporal resolution may be inadequate in that the images reconstructed with short scan data can suffer from blurring, streaking, or other imaging artifacts. This is due, fundamentally, to motion of the heart that occurs during this 240° acquisition, based on typical heart rates, gantry speeds, and the like. However, a quiescent period of the cardiac cycle occurs during approximately 90° to 130° of gantry rotation. It is desirable to increase temporal resolution in cardiac imaging applications, and in applications in general, where imaging artifacts may occur due to object motion over 240° of gantry rotation. It would be desirable to increase temporal resolution by a factor of up to 2, or even greater, based on typical heart rates, in order to improve images and reduce or eliminate image artifacts.
Temporal resolution could be improved by increasing the gantry speed and thereby decreasing overall acquisition time or modifying the hardware, such as adding additional sources and detectors. Artifacts may be reduced or eliminated because reconstruction occurs using data obtained over a smaller time period.
However, the gantry weight and other forces acting on the gantry limit the speed at which the gantry can operate. As is known in the art, load on the gantry increases generally as a factor that is squared with respect to gantry rotational speed. Therefore, after certain speeds, further reduction in the acquisition time typically requires more powerful x-ray tubes in order to achieve improved image quality. Thus there are life, reliability, and performance considerations to take into account, and it is highly nontrivial to maintain stability and functionality of components on the gantry at increased gantry speeds.
Another technique to improve temporal resolution includes a dual-tube/detector configuration. In such a system, two tubes operate simultaneously, thus decreasing overall acquisition time and increasing the temporal resolution as compared to a single source system. While resolution is improved, it comes with an associated increased that can be prohibitive. In addition, space limitations on the gantry can restrict placement of two x-ray tubes and two full-FOV detectors in a single compact gantry. Thus, the second detector often covers only a fraction of the desired scan FOV. Further, a dual-tube/detector CT system can include significantly more utility resources (i.e., coolant flow, electrical) when compared to a single tube system.
Thus, imaging suites containing such systems sometimes need significant and costly upgrades to provide additional the additional utility resources. Additionally, with an increased number of operational components, reliability of the overall system may be compromised because of the doubling in the number of primary components (i.e., tube, detector, and DAS). Thus, though such a system may improve temporal resolution, the increased temporal resolution comes at the cost of increased initial system expense and cost of ongoing operation, costly upgrades, and possibly reduced reliability when compared to a single source system.
It is also known that other imaging modalities, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), can also benefit from increased resolution to improve blurring and other image artifacts due to cardiac or respiratory motions. Such blurring may be caused by inadequate data acquisition during a given acquisition, or may be caused by an inordinate amount of time to obtain tomographic imaging data.
Thus there is a need for a system and method that minimizes motion blurring in tomographic imaging in a cost-effective and overall efficient manner without the added costs associated with doubling the hardware.